[gpaw-users] problem with the vacuum issue of graphene

[謝其軒]

Dear all,
When I calculate the band structure of graphene.
I set the distance between two sheets as 15~29 angstrom
It turns out that the shape of the band structure changed in the high
energy region(30~40eV)
in the gamma-K direction.

I thought the distance of the vacuum would not change the band structure.
But the result is not so.
Here's the input which I use to calculate the ground state.
======skip the import part
a=1.42
c=27
h = cutoff2gridspacing(100 * Rydberg)
atoms = Atoms('C2',[                  # Generate graphene AB-stack
structure.
              (1/3.0,1/3.0,0),
              (2/3.0,2/3.0,0),
              ],
              pbc=(1,1,1))
atoms.set_cell([(sqrt(3)*a/2.0,3/2.0*a,0),
                (-sqrt(3)*a/2.0,3/2.0*a,0),
                (0.,0.,c)],
               scale_atoms=True)

calc = GPAW(xc='LDA',
            kpts=(25,25,1),           # The result should be converged with
respect to kpoints.
            h=h  , # 0.2,
            basis='dzp',              # Use LCAO basis to get good
initialization for unoccupied states.
            nbands=70,                # The result should also be converged
with respect to bands.
            convergence={'bands':60}, # It's better NOT to converge all
bands.
            eigensolver='cg',         # It's preferable to use 'cg' to
calculate unoccupied states.
            occupations=FermiDirac(0.05),
            txt='out_gs.txt')

atoms.set_calculator(calc)
atoms.get_potential_energy()
calc.write('graphene.gpw','all')
=========

BR,
chi-hsuan
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